Microstructures in β-Mg_1.8Fe_0.2SiO₄ experimentally deformed at transition-zone conditions

The microstructures that develop in experimentally deformed β-Mg_1.8Fe_0.2SiO₄ have been investigated by petrographic and transmission electron microscopy to determine the deformation mechanisms that are active under transition-zone conditions. Advances in the application of the multi-anvil apparatus to deformation studies allowed us to deform β-Mg_1.8Fe_0.2SiO₄ at pressures of 14 GPa and greater and 1450°C. San Carlos olivine was transformed to β-phase and allowed to relax at an average strain rate of 1 × 10^-5 for 3 h and subsequently deformed at a constant strain rate of 1 × 10^-4. The olivine to β-phase transformation and deformation result in a bimodal grain size with the large grains having a preferred orientation. Deformation by dislocation creep and subgrain formation causes grain size reduction and nearly identical microstructures in the relaxed and high strain-rate samples. The dominant slip systems involved in the deformation of polycrystalline β-phase appear to be (010)[100] and (010)[001]. Slip in the [001] direction occurs by the glide of disassociated dislocations and produces Shockley-type stacking faults. The anisotropy that develops from the olivine to β-phase transformation under stress and subsequent dislocation creep may have profound effects on dynamic processes in the transition zone.